Many consumer electronic devices, such as smart-phones, lap-top computers, tablet computers and so on include a camera function for capturing still images and video. The camera function is typically provided by an image capturing module installed in the device.
These image capturing modules often include an auto-focus function. The auto-focus function enables a process to be performed whereby the focal length of the image capturing module is adjusted without direct input from the user so that a scene that a user is trying to capture an image of is in focus. The auto-focus process is typically performed by stepping through a series of focal positions (i.e. focal lengths), frame by frame, and analyzing each frame to determine a “sharpness” statistic (i.e. a focus statistic) captured by the image capturing module in each frame. A comparison of these statistics enables an optimal focal length to be identified and the focal length of the image capturing module to be adjusted accordingly.
Using this technique at least one frame is captured at several focal positions. It can often take many frames to determine the optimal focal length which results in a poor auto-focus convergence time (i.e. the time taken for the image capturing module to identify and move to the optimal focal length). A user may find this inconvenient particularly when attempting to capture an image of a scene that is changing rapidly.
Accordingly, it is desirable to reduce the auto-focus convergence time.